Why Georgia May Be Bluer Than It Appears

For all the hype about “Purple Texas,” the real front in the Democratic demographic offensive is Georgia.

No other plausibly competitive state has seen a more favorable shift for Democrats in the racial composition of eligible voters over the last decade. The pace of demographic change is so fast that Michelle Nunn, a Democrat, is locked in a tight race against the Republican David Perdue for an open Senate seat — even with an off-year electorate that is favorable for the G.O.P.

The pace of demographic change might even be fast enough to outpace the polls.

Polls show Mr. Perdue leading Ms. Nunn by about three percentage points. The various Senate models, including The Upshot’s, give Ms. Nunn about as good a shot — roughly 20 percent — of winning as Democratic incumbents in Louisiana, Arkansas and Alaska. But there’s reason to wonder whether her chances are better still: Recent polls are most likely underestimating the share of voters who are black, along with Ms. Nunn’s share of the vote.

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Michelle Nunn, the Democratic candidate for Senate from Georgia, is locked in a tight race with David Perdue.CreditErik S. Lesser/European Pressphoto Agency

The best evidence is that most recent polls are based on an electorate that’s no more diverse than in 2010, the last midterm election. In some polls, black voters represent an even smaller share of the electorate than in 2010. But Georgia has clearly become less white in the last four years.

According to data from the Georgia secretary of state, the 2010 electorate was 66.3 percent white and 28.2 percent black. Since then, the white share of registered voters has fallen, to 58 percent from 62.6 percent. White voters turn out at somewhat higher rates than other voters in midterm elections, so we should expect the white share of the actual vote to be a little higher. Combining the data on registered voters with census data on the voter-eligible population, I expect the 2014 electorate to be about 64.2 percent white and 28.8 percent black. (Ms. Nunn is expected to win at least 90 percent of the black vote.)

Yet the last four nonpartisan polls that released demographic data showed an electorate that’s 65.7 percent white and 25.7 percent black. Those polls show Mr. Perdue ahead by 3.3 points, but they would show something closer to a dead heat if the likely electorate matched my estimates.

Why do the polls show an electorate that’s no more diverse than 2010, even though there has been considerable demographic change?

A bit of the explanation is that the polls are being weighted to census estimates that haven’t kept up with demographic change. Most traditional surveys sponsored by the news media take a random sample of adults and then weight the sample to demographic targets from the census for variables like race, gender and age.

The catch is that the census products aren’t completely up to date. The census itself is now four years old, and the most recent estimates for 2013 just came out last month.

The exact targets for demographic weighting don’t usually make a discernible difference in the polls. Neither does the age of the product, which is invariably a year or even a few years out of date. But Georgia is perhaps the single state where it would make a noticeable difference, because of the degree of racial polarization and the pace of demographic change.

How far behind are the estimates? Over the last five years, the white share of adults declined by about half a point a year. That starts to add up if a poll is weighting to the 2010 census, as SurveyUSA or NBC/Marist do. The white share of Georgia adults has probably dropped by about two points since.

Pollsters can choose to weight to more recent targets, like the American Community Survey, a large annual survey conducted by the Census Bureau. But the survey isn’t released until the year after sampling; the 2013 A.C.S. was released just a few weeks ago. The pollsters using the A.C.S. for weighting — like The Atlanta Journal-Constitution/SRBI and the CBS/NYT/YouGov poll — were using the slightly older 2012 survey. The 2013 A.C.S. is already a half-point more diverse than the 2012 version, and there’s another year of demographic change yet to be measured.

But this factor doesn’t fully explain the low estimated share of the black vote.

A few possibilities might explain the rest of the difference. One of those is a technique known as “trimming weights” — in which pollsters don’t weight underrepresented groups up to their targets because it would require individual respondents to be weighted too heavily.

Pollsters often choose to trim weights if a single respondent starts getting weighted more than a few times over. Often, this involves undersampled demographic groups that lean Democratic, like young and nonwhite voters. As a result, many surveys fall a bit short of their targets for nonwhite voters; it was one of the many reasons the Gallup poll fell short of its targets for nonwhite voters in 2012.

Another possibility is that the likely-voter screens — in which pollsters estimate who will vote and who won’t — are excluding too many black registered voters, either because the screens are too tight or because Democratic mobilization efforts aren’t yet underway.

One pollster has raised this possibility. Mark Schulman, chief research officer at Abt SRBI, noted that The Atlanta Journal-Constitution/SRBI poll applied a “fairly tight” likely-voter screen (eliminating people who said they were less engaged in the election, for example). Black voters represented 24 percent of the electorate in the poll, compared with 28 percent in 2010. Mr. Perdue led by four points; it would have been a dead heat if the black share of the electorate were four points higher.

Of course, the black share of the electorate might not stay as high in 2014 as the 28.2 percent share in 2010. Black turnout, for instance, might well go up, but white turnout might go up even more. But we can safely dismiss the possibility that the black share of the electorate will crash into the mid-20s — and that’s where several of the most recent polls put it.

None of this should be interpreted to mean that Ms. Nunn is the favorite, nor should it be interpreted to mean that she’s assured to do better than the polls suggest. Changes in voter sentiment, or the possibility that the polls aren’t accurately measuring voter sentiment in the first place, could easily cancel out any benefits that Ms. Nunn might accrue from a more diverse electorate.

Even if Ms. Nunn does win on Nov. 4, she probably won’t receive the 50 percent necessary to avoid a January runoff. I don’t hear very many Democratic pollsters or campaign operatives screaming about how Ms. Nunn is really in the lead, either.

But there isn’t much question that the most recent polls underrepresent the nonwhite share of the Georgia electorate. Even if there’s something else wrong with the polls that cancels it out, it’s another reminder of the mounting methodological challenges facing pollsters in an era when random sampling is no guarantee of a representative sample. Weighting isn’t always easy; neither are likely-voter screens.